Insulating coating for filaments and method of applying the same



Aug. 1932. J. KAUL 1,874,542 INSULATING COATING FOR FILAMENTS AND METHODOF APPLYING THE SAME Filed Sept. 24. 1929 INVENTOR Jo m/ KczuZ BY Hi5ATTORNEY Patented Aug. 1932 UNITED STATES PATENT OFFICE JOHN RAUL, O]?IRVIIIGTON, NEW JERSEY, ASSIGNOB TO ARCTUBUS RADIO TUBE COH- Pm, OFNEWARK, NEW JERSEY, A CORPORATION OF DELAWARE INSULATING COATING FOBIILAKENTS AND METHOD OF APPLYIITG THE Application fled September 24,1829. Serial No. 894,823.

This invention relates to coatings for filamentary wires and especiallyinsulating coatings used on the filaments of vacuum tubes.

It is generally desirable in vacuum tubes to place the filament asnearly as possible to the other elements of the tube in order to takeadvantage of all of the heat radiated from the filament or the electronemission where a filamentary cathode is used. Especially in a heatertype tube is it necessary to have the heating element as close to thecathode as possible so as to conserve the heat energy of the filamentand reduce to a minimum the time required to bring the cathode up toelectron u emitting temperature when the current is ofi the binder.

turned on. Where a bare filament is positioned in close proximity to thecathode any movement of the filament with respect to the cathodemayjcause the two to touch and thereby short circuit the tube. It hasbeen found advantageous to provide some insulating means between thecathode and filament to prevent such short circuiting and one way ofproviding such an insulation between the two elements is to coat it uponthe filament itself. Such an insulating coating for filaments has beenprovided by mixing an insulating oxide as for instance, aluminum ormagnesium oxide with a suitable binder coating this mixture upon thefilaments and heating to drive There are certain disadvantages whichfollow from this method of insulation, the first being that it isextremely difficult to drive off all of the binder and in some instanceswhere a little of one binder remains the conductivity of the insulationis increased thereby causing break down at relatively low temperatures.Another disadvantage of this type of insulation is that the particles ofinsulation materials are not in a molecular state but, even thoughground in a ball mill and made exceptionally fine, tend to loosen up andabsorb moisture so that the finished coating has a tendency to crumbleoff of the wireand geneous, extremely compact surface tightly associatedwith the metal base.

Another object of the invention is to provide 2. hard, compactinsulating coating for filaments capable of standing the temperatures oftube operation and manipulating of the filament before it is positionedin the tube without the coating crumbling or breakin down electricallyunder the potentials use Another object of the invention is to provide amethod of attaching the oxide of a metal to a wire without any of thedisadvantages arising from the use of binders.

Other objects and objects relating to methods of preparing the variousingredients of the coating and applying the coating to the wire will beapparent as the description of the invention proceeds.

Referring to the accompanying drawing, the figure therein illustrates ingeneral a coated filament in accordance with my invention.

The specific showing in the drawing illus trates a filament 1 upon whichis applied a coating 2. Thefilament comprises a length of wire of asuitable character to be heated by passing a current therethrough. Thecoatin is preferably an oxide as will now be more ully described. I 7

Among the oxides which form the best insulating material for use invacuum tubes are those of aluminum, beryllium, magnesium, calcium,chromium, and titanium. Of these metals I prefer to use beryllium oraluminum because the compounds are commercially obtainable and as goodresults have been obtained with aluminum the method of obtaining acoating of aluminum oxide will be described.

I first prepare a saturated solution of aluminum nitrate and apply anydesired number of even coatings of this solution upon the wire which ispreferably heated after each coating has been applied to dry thematerial in place. The action of heat on the aluminum nitrate breaks itup into aluminum oxide,

nitric oxide, and water, the last two being driven oil in va or formleaving a coating of aluminum oxi e in its molecular state. I have usedthis coating on a tungsten filament ess of the tube and cause thealuminum exit e to adhere very closely to the tungsten wire.

I have found that the degree of saturation of the aluminum nitratesolution is important as this determines the number of applications orlayers necessary for acoatingoi the desired thickness and to a certainextent the character of the coating after it is applied. If

vthe solution is weak, a large number of applications will be needed togive the necessary cross section. I And if the solution is. asupersaturated solution there is a tendency vfor the aluminum nitrate tocrystallize and cause lumps and uneven ridges on the wire surface.

A saturated solution'having a specific gravity of 1.4 at 27 C. has beenfound to give good results. With1a6 mil wire I- have found that aboutcoatings of this solution are necessary to raise the diameter to from 13to 15 mils.

To aluminum nitrate solution I addyfrom 3 to 10% by weight of'silicondioxide'which melts below the filament temperature during the exhaustingof the vacuum tube, thereby forming themeans'to integrate the aluminumoxide more closely to the wire. It is important to get enough of thesilicon dioxide to melt the coatingso that it closely adheres to thewire and not enough to lower the dielectric.- o

After a filament is inserted in a vacuum tube'the tube is exhausted atwhich time the filament is heated to a. temperature in the neighborhoodof 600" to 900 C. 'or toa dull red heat. I have found that this heat-'ing of the filament where aluminum nitrate has been coated thereonwithout any silicon dioxide causes a softening of the coating whichrenders it crumbly and undesirable to be used in a vacuum tube, andalthough I do not know the cause of this softening I have found that theaddition of the silicon dioxide prevents it and hardens the coating intoone that is extremely difficult to chip or crumble.

While I have described the invention in connection with aluminum any ofthe metals listed above may be used, and while I have specified silicondioxide as preferable to mix with the material applied to the wire, anymetallic oxide which is a good insulator and has a meltingv point at orbelow a dull red hea-t may be used for this purpose}:

l he seen that I have forming a metallic The .e, inexpensive, andproduces coating for the filament which inble or break ofi when thefilahent in desired shape' for positiening in a vacuum tuhe, nor will itcrumble or break down when the filameiia is in actual service therebygreatly lengthening-the life of the vacuum tubes sin the invention,especially those of t e9. 1' t Modifications e the in without depemtingfrom th nticn may be made spirit thereoi and I do not desire to limitthe invention to crystallization temperature of ther ungsten" of saidWire, heating said wire after each layer of the solution hasbeen appliedto reduce the aluminumnitrate to aluminum 'ox ide, and subsequentlyraising the temperature of -said wire to a point between the meltingpoint of the first mentioned oxide and the crystallization temperatureof said wire.

2. The method of coating a tungsten wire with a hard, homogeneousinsulating coat ing which comprises applying layers of a, solution ofaluminum nitrate and a small-percentage of silicon dioxide to the wire,heating t ewire after each successive application to chemically changethe aluminum nitrate to aluminum oxide, and subsequently heating thewire to a temperature above the lmelting point of silicon dioxide butbelow the crystallization temperature of tungsten.

3. The method of forming a metallic oxide ona filamentary wire whichcomprises coating'the wire with a plurality of layers of a saturatedsolution of aluminum nitrate having a specific gravity of 1.4 at27Chmixed with from 3 to 10% by weight of silicon dioxide, heating thewire after each successive layer has been applied to reduce the aluminumnitrate to aluminum oxide, and subsequently heating the wire to atemperature above the melting point of silicon dioxide but below thecrystallizationpoint of said filamentary wire.

' In testimony whereof, I afix my signature.

' JOHN KAUL.

